1. Field of the Invention
The present invention relates to a method enabling structured documents to be divided into several parts.
It applies particularly but not exclusively to handling, transmitting, storing, and reading structured multimedia documents, digital or video images or image sequences, movies or video programs, and more generally to any transfer of said documents between processor units interconnected by data transmission networks, or between a processor unit and a storage unit, or indeed between a processor unit and a playback unit such as a television set if the document is a video program.
2. Description of the Related Art
More and more frequently, documents handled and transmitted in this way contain a plurality of different types of data integrated in a structure. A structured document is a connection of data sets each associated with a type and attributes, and interconnected by relationships that are mainly hierarchical. Such documents use a markup language such as Standard Generalized Markup Language (SGML), Hypertext Markup Language (HTML), or Extensible Markup Language (XML), serving in particular to distinguish between the various subsets of information making up the document. In contrast, in a “linear” document, the content information of the document is mixed in with layout information and type information.
A structured document includes markers for separating different sets of information in the document. For SGML, XML, or HTML formats, these markers are referred to as “tags” and have the form “<XXXX>” and “</XXXX>”, the first marker marking the beginning of a set of information called “XXXX”, and the second marking the end of said set. A set of information may itself be made up of a plurality of lower-level sets of information. Thus, a structured document presents a tree or hierarchical structure schema, each node representing a set of information and being connected to a node at a higher hierarchical level representing a set of information that contains the sets of information at lower level. The nodes situated at the ends of branches in such a tree structure represent sets of information containing data of predetermined type, themselves not suitable for being resolved into subsets of information.
Thus, a structured document contains separation markers represented in textual or binary data form, said markers defining information sets or subsets that can themselves contain other subsets of information defined by the markers.
A structured document is associated with a structure schema defining the structure in the form of rules together with the type of information in each set of information of the document. A schema is constituted by nested groups of information set structures, these groups possibly being ordered sequences, groups of alternative elements, or groups of necessary elements, ordered or not ordered.
At present, when a structured document is to be transmitted, it is initially compressed so as to minimize the volume of data to be transmitted. For best efficiency in such compression processing, the document structuring data is also compressed, given that the recipient of the document is assumed to know beforehand the structure schema of the document and to be able to use the structure schema to determine at all times what information set is about to be received. It is therefore essential for the structure of the document as transmitted to correspond exactly to the structure schema that the recipient of the document intends to use for receiving and decoding the document, since otherwise the recipient cannot determine the type of data that has been transmitted and is thus incapable of decoding the data and of reconstituting the original document.
Unfortunately, structured documents for transmission are tending to become more and more voluminous. Proposals have been made, for example, to transmit or broadcast complete descriptions of movies or TV programs in this way.
In this context, if a transmission error should occur while a document is being transmitted, the recipient of the document may no longer be able to determine which subset is being transmitted, in which case the entire document needs to be transmitted again. Furthermore, if it is desired to transmit a movie sequence and display it simultaneously on a screen, it can be necessary to comply with periods of time for transmitting the various elements of the sequence. Certain elements of the sequence must also be capable of being transmitted several times over so as to enable a recipient who was not connected at the beginning of the transmission of the sequence to receive and display the end of the sequence.
It may also be necessary to replace a portion of a document by another, these two portions having the same structure schema.
The solution which consists in retransmitting the entire document leads to a considerable increase in the volume of information that needs to be transmitted. It is therefore desirable to be able to divide a document into a plurality of portions which are transmitted separately. It turns out that present transmission methods are not suitable for transmitting a document in part only.